1,4-Dioxane

1,4-Dioxane has been used as a stabilizer for solvents, in particular 1,1,1- trichloroethane (TCA), and a solvent in its own right, as well as in a number of industrial and commercial applications (ATSDR, 2007; NTP, 2011).

The chemical causes cancer in laboratory animals and is reasonably anticipated to be a human carcinogen, first listed in the Annual Report on Carcinogens in 1981 (NTP, 2014). In 1988, 1,4-dioxane was added to the list of chemicals known to the state to cause cancer [Title 27, California Code of Regulations, Section 27001]. US EPA also considers it to pose a cancer risk (US EPA, 2010; 2013).

1,4-Dioxane is considered an emerging contaminant, and one for which additional monitoring may be appropriate.

Notification Level

The drinking water notification level for 1,4-dioxane is 1 microgram per liter (μg/L). Certain requirements and recommendations apply to a water system if it serves its customers drinking water containing a contaminant greater than its notification level. The response level, the level at which removal of the source from service, is 35 μg/L.

In 1998, the Drinking Water Program, now the Division of Drinking Water (DDW), established its initial notification level at 3 μg/L, based on a US EPA (1990) drinking water concentration that corresponded to a 10-6 theoretical lifetime cancer risk. Later, in 2010, US EPA revised its 1,4-dioxane risk evaluation, such that a 10-6 risk level corresponds to 0.35 μg/L (US EPA, 2010a, 2010b, 2013). We revised its notification level to the 1-μg/L level in November 2010.

The notification level is slightly greater than the de minimis (1 X 10-6) level commonly used for notification levels based on cancer risk, reflecting difficulty in monitoring 1,4-dioxane at very low concentrations.

Analytical Methods

The recommended laboratory reporting limit for 1,4-dioxane is 1 μg/L, the same as the notification level. The reporting limit is similar to the detection limit for purposes of reporting, (DLR), which is established in regulation for chemicals with maximum contaminant levels. The DLR is the level at which the DDW is confident about quantification being reported.

The prior recommended reporting limit was 3 μg/L, which was equivalent to the previous notification level. Some water systems were already using laboratories capable of measuring 1,4-dioxane at very low levels using Method 522. Some laboratories were also able to detect 1,4-dioxane at very low levels using hazardous waste Method 8270c.

DDW recommends that water systems' laboratories use the more sensitive analytical method to enable better characterization of the presence of 1,4-dioxane in drinking water sources.

Findings in California Drinking Water

In 1998, we were notified about a 1,4-dioxane detection in a groundwater well; subsequently over the past decade, it has been found in a number of wells, mostly in southern California. In 2002, the presence of 1,4-dioxane in wastewater became problematic for a groundwater recharge project in southern California, prompting a need for additional water treatment.

Drinking water systems are not required by state regulations to monitor for 1,4-dioxane. Nevertheless, because of concerns about possible contamination, a number of systems have been directed by DDW to or have chosen to sample their supplies for 1,4-dioxane.

Detections included in the accompanying spreadsheet should not be considered to reflect 1,4-dioxane in drinking water served to customers, since water from the listed sources may be blended, treated, or not used to provide drinking water. For more information about specific drinking water supplies, see public water systems' annual Consumer Confidence Reports.

Table 1. Drinking water sources and systems reporting a peak detection of 1,4-dioxane
at or greater than 1 microgram per liter (2012-2015)

County (ID Number)

Number of Sources

Number of Systems

Peak Concentration
(μg/L)

Los Angeles (19)

111

36

29.3

Monterey (27)

1

1

3

Orange (30)

23)

13

9.7

TOTAL

135

50

-

For this table, we've used monitoring results reported to the DDW and extracted from the water quality monitoring database. "Sources" are those with two or more reported detections, and include active and standby wells. Reported detections, are included here (Excel); non-detects are not included in the spreadsheet. The water quality monitoring database is available here.

NOTE: These data should be considered draft, since they will change with subsequent updates.